Is mulching an efficient way to control weeds? Effects of type and amount of crop residue in rainfed rice based cropping systems in Madagascar

Weeds are a major constraint to crop yields in tropical production systems, especially for smallholder farmers who cannot afford to purchase herbicides. It has been argued that the practice of mulching with crop residues can suppress weeds in conservation agriculture systems. However, few data are available on the effect of crop residue mulching on weed infestation. In this study we quantified the effect of increasing amounts of surface crop residues on weed emergence, weed biomass production and rice yield. The experiment was conducted during four growing seasons in the Lake Alaotra region, Madagascar. Two types of mulch were applied on a no-tilled soil, Stylosanthes guianensis and a mixture of maize and Dolichos lablab, with different amounts ranging from 0 to 45 Mg dry matter ha−1. Weed emergence was measured every week from the day of the first rain that triggered weed germination to 100 days thereafter, and weed biomass was monitored at four dates during the cropping season. Our results show that weed emergence and weed biomass decreased with increasing amount of residue. More than 10 Mg ha−1 was, however, needed to significantly reduce weed emergence and weed biomass as compared to the bare soil treatment without surface residues. Rice grain yields decreased by 16% for an increase in weed biomass of one Mg ha−1. Our results indicate that mulching is not a viable option of weed control for smallholder farmers, given the low amounts of residue currently retained on their fields, which is at best about 4 Mg ha−1 in the case of a stylosanthes crop and about 5 Mg ha−1 in case of dolichos intercropped with maize.